1. Field of the Invention
The invention relates to a communication apparatus for establishing bidirectional communication among a plurality of moving objects and, more particularly, to a vehicular communication apparatus suited to be installed in a vehicle.
2. Description of the Related Art
A vehicular control operation for safety driving is often performed on the basis of pieces of information that are collected by means of various sensors installed in a vehicle, a radar unit for detecting an obstacle around a vehicle, or the like. Also, pieces of information on traffic jam and the like are often provided to a running vehicle through communication between a road-side equipment disposed close to a road and an on-vehicle equipment installed in a vehicle (road-to-vehicle communication).
In recent years, communication among vehicles (hereinafter referred to as “vehicle-to-vehicle communication”) has been proposed. Namely, the idea of performing a control operation for preventing accidents through cooperation of vehicles based on exchange of information or the idea of relaying information obtained by a certain vehicle from a road-side equipment to other vehicles so as to make it possible to indirectly acquire information provided by the road-side equipment without directly establishing road-to-vehicle communication has been discussed.
In vehicle-to-vehicle communication, each vehicle is provided with a radio transmitter-receiver that is designed to transmit a radio signal including information on an own vehicle and to receive a radio signal including information on a foreign vehicle. Vehicle-to-vehicle communication is established between one own vehicle and one or more foreign vehicles. Therefore, under a circumstance where a plurality of vehicles run while crowding around one another, the following two problems are caused. One of the problems is that each of the vehicles cannot efficiently acquire required information on any one of the other vehicles. The other problem is that an increase in the amount of information to be processed leads to an increase in the burden in performing processings.
In order to solve the problems mentioned above, it has been proposed to detect various situations in and around an own vehicle as situational data and to change a degree of frequence of transmission of an information signal in accordance with the situational data, as disclosed in Japanese Patent Application Laid-Open No. 2000-311294. This art is characterized by changing a degree of frequence of transmission of an information signal for the purpose of efficiently acquiring required information on one or more of many foreign vehicles that exist around an own vehicle.
In a vehicle-to-vehicle communication system of the related art, all pieces of possibly useful information are transmitted or received among vehicles participating in vehicle-to-vehicle communication. Hence, the amount of information included in each signal to be transmitted or received increases, so that the burden in processing the signal on the reception side increases. Under a circumstance where many vehicles run while crowding around one another, the following problem is caused. Namely, radio waves transmitted from a plurality of vehicles are scattered about in a frequency bandwidth assigned to vehicle-to-vehicle communication, so that the quality of communication deteriorates.
On the other hand, as disclosed in the aforementioned publication, the idea of changing a degree of frequence of transmission of a signal in vehicle-to-vehicle communication on the basis of various situations around a moving object such as a frequency utilization factor and a distance from an intersection serves to alleviate the burden in performing processings on the reception side and to make an improvement in frequency utilization factor. However, under an environment where a plurality of vehicles run while crowding around one another, the following problem is caused. Namely, the degree of frequence of transmission of a signal is so changed as to decrease, so that the usefulness of vehicle-to-vehicle communication is lessened. Especially under an environment where a plurality of vehicles run while crowding around one another, the degree of necessity for vehicle-to-vehicle communication is intrinsically high. Reduction of the degree of frequence of transmission of a signal under a situation as mentioned above cannot always be considered to be an advantageous measure.